Human respiratory syncytial virus (hRSV) and human metapneumovirus (hMPV) are non-segmented negative- strand viruses (NNSV) and are the leading causes of acute respiratory tract infections in infants worldwide. In addition, hRSV is a significant cause of disease in elderly populations and can often be fatal for patients with compromised immune systems. Currently no vaccines are available, and existing therapeutics (e.g., ribavirin, immunoglobulin, or anti-hRSV monoclonal, Synagis) exhibit poor efficacy and present safety concerns. The development of safer more effective therapeutics is a major unmet medical need. The goal of this project is to address this need by discovering and developing inhibitors of hRSV and hMPV RNA synthesis for therapeutic use by targeting the interaction between the viral nucleoprotein (N) and the viral P protein, a cofactor for the viral polymerase (L). This interaction is critical for viral RNA synthesis; in cells infected with NNSVs, an L-N complex is required for replication, and P mediates interactions between L and the N-RNA template. The strategy is to build and apply biochemical screens for inhibitors of the hRSV and hMPV N/P interaction based on fluorescence polarization. This approach is based on a successful anti-Ebola virus screening effort carried out by this team to identify inhibitors of the interaction between the Ebola nucleoprotein (eNP) and the Ebola P protein equivalent, known as eVP35. Development and application of a primary fluorescence polarization assay (FPA) followed by secondary assays including a counter-screen FPA based on an unrelated interaction resulted in the discovery of six specific eVP35/eNP interaction inhibitors with IC50 values ranging from 1 M to 35 M. Two of these compounds inhibited Ebola RNA synthesis in a cell based assay known as a minigenome replication assay. In Phase I, these efforts will be extended to target this conserved viral interaction by focusing on hRSV and hMPV, which are of broad clinical importance. Primary FPA screens for inhibitors of the hRSV and hMPV N-protein interactions with fluorophore-labeled peptides from the corresponding P-proteins will be developed. In addition, biochemical (e.g., biolayer interferometry, BLI) and cellular (e.g., split luciferase) secondary assays with orthogonal read-outs will be constructed to validate initial hits and to assess cellular permeability and mechanism of action. The primary and secondary assays will be applied to >400,000 diverse compounds. Confirmed potent, selective inhibitors will be validated by determining their ability to inhibit infectious viral assays and by ensuring that they are not cytotoxic. In vitro ADME assays and preliminary SAR will prioritize analogs for further optimization. Strengths of this proposal include the productive, collaborative research team; highly sensitive, homogeneous FPA screens; FPA counter-screens to rapidly recognize and eliminate false positives; potential to identify broad inhibitors targeting hRSV and hMPV; and cellular assays to establish the target-specific function. In Phase II, priority validated inhibitors will be chemically optimized into lead compounds for efficacy and toxicity testing in animal models.